Radio transmitting system



g- 1932- H. ROMANDER RADIO TRANSMITTING SYSTEM Filed Jan. 13, 1930' K40 INVENTOR Romandcr' ATTORNEY I Patented Aug. 23, 1932 V uNi Eo sT r sf HUGO nomennnn, or mm ,ALro, CALIFORNIA, Assrenon *ro FEDERAL TELEGRAPH COMPANY, or SAN FRANCISCO, CALIFORNIA, A oonroanrxonor CALIFORNIA 'nADIo rnnnsnrrrnve SYSTEM Application. filed January 13, 1930. Serial No. 420,312.

This invention relates to high frequency carrier wave telegraph transmitting systems and particularly to short wave radio transmitters. j i

An object of the invention is to secure constancy in the frequency of the carrier Wave generated in such a system.

Regulation of the frequency of radio transmitters has been a serious,,practi'cal problem for many years, particularly in short wave transmitters where-the frequencies of different channels differ by an extremely small per centage although the differences as expressed in terms of wavelengths or cycles may be relatively great. Frequency drift occurs par: ticularly as a transientv condition following the starting of oscillations in a, vacuum tube oscillator and is therefore, especially annoying in radio telegraph transmittersv in which keying is accomplished by starting and stop.- ping the oscillator. i i

It has been discoveredthat heating, and consequent expansion, of'the grid is one of the chief causes oftransient frequency drift, in accordance with this invention such drift is reduced by maintaining the grid of the tube at a uniform temperature at all times regardless of whether or not it is oscillating. This uniform grid temperature is maintained by applying to the grid during non-signaling intervals a current whichhas the same heating eflect thereon as the oscillatory current generated when the key is closed. H

The invention will now be described in de.-' tail with the aid of the accompanying drawing in which:

Figs. 1, 2, and 3 are schematic circuits showing three alternative methods of heating the grid of a vacuum tube during non-' signaling intervals. 1

In Fig. 1, a vacuum tube 1, having a cathode 2, grid 3 and anode 4, has a frequency'determinating, tuned output circuit comprising a variable condenser 14 and inductance 15.

" The inductance15 is coupled to an inductance 16 connected to a transmitting antenna 17 The lowerterminals of the tuning condenser 1e and inductance 15 are connected to the cathode 2'through a by-pass condenser 13.

Anode potential for the tube is supplied from 1 a battery or other source of potential through contacts of relay 20, and radio frequency choke coil19. Relay 20 is connected in the keying circuit, which includes battery 24. and lower contacts 22 of key 23. I

The input, or cathode-grid circuito-f the tube 1 comprises an inductance and a radio frequency by-pass, condenser 5 in series be tween the cathode and grid. In a circuit of thistype sufiicient energy is transferred from 1 the output circuit to the inputcircuit through the grid-anode capacity of the tube to pro 1 duce continuous oscillations,at shortwavelengths. Shunted across condenser 5 1s a source of low frequency current comprising being determined by the characteristics of the tube and the conditions under which it is operated. The secondary voltage may be varied within limits by adjusting a rheostat 10 in the primary circuit. To prevent high potentials of radio frequency from being impressed on the windings of transformer 7, bypass condensers 6 and 42 may be shunted across the secondard winding and between the primary and secondary terminals, respectively, as shown; f

In operation, the tube 1 is energized to pro:

duce, radio frequency oscillations by pressing key 23, which closes the circuit between battery 24 and relay 20, causing the latter to operateand connect the anode battery 25 between the cathode and anode of the oscillator tube. When the tube 1 is oscillating, the grid 3 becomes alternately positive and negative with respect to the cathode 2, and during intervals when the grid is positive it attracts compensating heating efiect, on the grid when tube 1 is not oscillating, upper contact 21 is provided on the key 23 which closes a circuit to energize relay 26, at the same time relay is released. Relay 26, in operating, closes contacts 27 and 28, which connect the source of 60 cycle alternating current 41, to pri-.

mary winding 9 of transformer 7. The resultant alternating voltage induced in winding 80f transformer 7 is applied between the grid and cathode of the oscillator tube and produces a rectified grid current in substantially the same-manner'as does'the radio fre- 1 'quency' potential applied to the grid circuit when the tube is oscillating. "By varying the potential of the 60 cycle current applied to rent at all times, a direct current ammeter 1 1 the grid, as by varying the rheostat 10 in the primary circuit of transformer 7 a grid heating effect can be produced which is substantially the same as that of the high frequency current produced by the oscillator. Because'the grid of the oscillator tube requires an appreciable time to change its temperature, itis unnecessary that the grid circuit be energized with the 60 cycle heating current each time contact 21 on key 23 is closed. It is sufiicient if power is applied to the grid circuit during actual pauses in signaling, when the tube is inactive for several seconds. Furthermore, it is undesirable that the grid heating current be applied oftener than is necessary because of the excessive wear and resultant rapid deterioration of contacts 27 and 28. To this end, relay 26 is preferably of the daslrpot type which operates only in response to a closure of upper key contact 21 in excess of a predetermined time. The relay must, however, be adapted to release quickly in order thatrthe grid heating circuit may be disconnected following opening of contacts 21, before relay 20 operates to energize the tube for the production of oscillations. The dash-pot mechanism should, therefore, be designed to permit rapid motion in one'direction, while restraining it in the other. 7 7

During intervals of rapid keying, while relay 26remains unoperated, it has been found necessary for satisfactory operation to short out the secondary winding 8 of transformer 7 by means of auxiliary contacts 29 on relay. 26. The reason for this is that when the tubeis oscillating, more or less rectified current flows in the grid circuit, whereas, during the short intervals when the key 23 is up and the tube is not oscillating no grid current flows. Were contacts 29 not provided, this rectified grid current would have to pass through secondary Winding 8, which since it has a Very high inductance, would. oppose the building up of a 1 current, and would result in a transient negative bias on the grid 3 which would of itself cause an undesirable shift in frequency. Furthermore, even were the artificial grid heating current applied each and every time that the value of the rectified grid current, and

therefore, the amplitude of the direct current flowing in the grid circuit is a rough measure of the heating effect beingproduced. To indicate the amplitu'deof'the rectified grid-curis provided in the grid return path.

The circuit of Fig. 2 is similar to that of Fig.1, except that it. involves the use of a simpler relay structure for applying the grid heating current. In thiscase a source of current 33, which is shown as a direct current generator but may be an alternating current source, is adapted to be connected, through the armature and front contact31 of relay 30-,

between the cathode 2 and grid 3 of tube 1. r

The keying circuit differs from'that shown in Fig. 1 in that the key 23 has only one contact 22 which closes a circuit to operate the keying relay 20 and the grid heating control relay 30, simultaneously. When the key 23 is in normal position the armature of relay 30 lies against its back contact 32, thus connecting the negative terminal of source 33' to cathode 2 through grid ammeter 11, since the positive terminal of source 33 is permanently connected' tothe-lower terminal ofinductance 50. This completes a direct current circuit from the cathode to the grid,through source 33. When key 23 is closed, relay 20 applies oper- 'atingpotential to the anode from source 25, and relay 3O disconnects source 33- and completes another direct current circuit from the lower terminal of inductance 50 through front contact 31 and the armature offrelay 30, through ammeter 11 to the cathode. The advantage. of this arrangement over'that dis closed in Fig. 1 is that relay 30, having a simple contact structure, may be of the vacuum relay type in which the contacts are enclosed in an evacuated space. Such relays are relativelydurable and give satisfactoryservice under continuousoperation- As pointed out in connection with Fig. 1,-however, it is not necessary to the successful operation. of the system thatrelay 30- close the grid heating circuit during veryshort intervals between signals, and relay 30 may therefore be of the slow release type so that contact 32'is closed only after a predetermined non-signaling interval. Fig. 3 depicts a modification of the circuit disclosed in Fig. 2 in which'the source of anode potential 25 is used to supply both the anode current and the grid heating current. In'this circuitthe keying relay-38 is provided with a back contact 39. When signals are not being transmittedthearmature of relay 38 is closed on its back contact 39, thus closing a circuit from the positive terminal of the source of anode potential 25 through a potentiometer comprising resistor 36 and 37 to the cathode 2. The resistance of the potentiometer is preferably chosen substantially equal to the average resistance of the anode circuit of the tube so that the current drawn from source 25 is uniform during both sig naling and non-signaling intervals. Such a provision for maintaining a constant load on the source of anode potential is particularly useful to prevent sudden surges of current and voltage when the source of anode potential is a high impedance rectifying and filtering system excited by alternating cur rent. The adjustable resistance 37 of the potentiometer is adjusted to apply such a potential to the grid 3 as will produce a desired grid heating effect. l/Vhen signals are transmitted by pressing key 23, relays 34 and 38 operate simultaneously, relay 38 disconnecting the source of anode potential 25 from the potentiometer and connecting it, over front contact 40, to the anode circuit of the tube. At the same time, relay 34 closes its armature on contact 35 to short circuit the resistor 36 and provide a low impedance path from the lower terminal of inductance 50 through ammeter 11 to cathode 2.

Although not necessarily limited thereto, this invention is chiefly applicable to oscillators comprising high power tubes with water cooled anodes which are maintained at substantially uniform temperature during both operating and non-operating periods by the water circulating therethrough. In such tubes the cathode and anode temperatures are substantially uniform regardless of whether the tube is oscillatin or not, and by providing the above descri ed means for maintaining a uniform grid temperature, practically all variations in the inter-electrode capacities of the tubes may be eliminated.

I claim:

1. The method of increasing the frequency stability of a vacuum tubeoscillator which comprises maintaining the grid of the tube at a substantially uniform temperature over a period of time including oscillating and non-oscillating intervals. i

. 2. The method of increasing the constancy of'the frequency of a vacuum tube oscillator p in a radio transmitter which comprises maintaining the grid of the tube at a substantially uniform temperature at all times.

3. In a radio transmitter comprising a vacuum tube oscillator with means for starting and stopping the cathoderanode current in said tube, the method of improving the frequency stability of the oscillator which comprisesprodu'ci'ng the same heating effect on the grid of the tube when the anode circuit, of the tube is tie-energized as when it is energized. V r r a. A radio transmittercomprising a vacu: um tube oscillator having input and output radio frequency circuits coupledtogether, a source of cathode-anode current, a source of B5 cathode-grid current, circuits for connecting said sources of current between said cathode and anode and cathode and grid, respectively, and keying means for simultaneously interrupting one circuit and closing the other. a

5. In a radio telegraph transmitter, a vacuum tube oscillator, a source of anode potential therefor, keyingmeans for controlling theapplicatio-n to the anode of said vacuum, tube of potential from said source, a radio- 98 frequency path between the cathode and grid and'means in said path preventing the flow of current of less thanradio frequency, a second path between said cathode and grid, a

source of potential, and means responsive to 2100 said keying means for applying said potential to said grid over said second path during non-signaling periods.

6. In a radio transmitting system comprising a vacuum tube oscillator adapted to pass appreciable space current only when signals are being transmitted, the method of increasing the constancy of the frequency of opera tion which comprises energizing the cathodegrid circuit of said tube with current of .a i

non-radiating frequency duringjnon-signal hng periods.

7. In a high frequency wave transmlttlng system a vacuum, tube oscillator, lnput and output circuits therefor, a tuned frequency I controlling circuit in said output circuit, means for energizing said output circuit whereby radio frequency oscillations may be generated by said tube, means for applying an el ectromotive force of less than radio fre- U20 quency to said input circuit, and switching means whereby said electromotive force is applied to the input circuit of said tube only during periods when said output circuit is not energized.

1 8. In a radio transmitter an oscillator comprising a vacuum tube having a cathode, grid, and anode, a tuned circuit associated therewith and serving, in combination with the *tube constants, to determine the frequency of I80 bscillation, a source of currentjfor energizmg said cathode, a source of anode potential, a source of alternatlng current of sub-radlo frequency, switching means for applying said anode potential to said anode, second switching means for applyingsuch sub-radio frequency alternating current to the grid of said tube and'keying means for simultaneously closing one of said switching means and opening theother.

:9. In a hign frequency transmitting system comprising an oscillatingvacuum tube having a cathode, grid and anode and adapted to pass appreciable space current only when signals are being transmitted, the

method of reducing transient variations in the frequency of operation which comprises energizing the cathode-grid circuit of said tube whereby a grid heating current is produced during non-signaling intervals.

10. In ahigh frequency oscillation generator, a vacuum tube having a cathode, grid and anode, a source of anode potential,

switching means for applying said potential to said anode for controlling the production of'oscillations, and means for applying a positive potential to said grid during peri' the grid is maintained at substantially a constant temperature at all times. I 11. In a high frequency oscillation generator, a vacuum tube containing a cathode adapted to emit electrons, an anode, and a grid positioned therebetween, an input circuit and an output circuit, a frequency determining circuit including as a determining element thereof the inherent capacity between said grid and the other electrodes,

y and.

perature-of saidcontrolelectrode duringnonoperating intervals of said'tube, whereby the temperature of saidcontrol electrode is kept substantially constant during both operating and non-operating intervals.

., In testimony whereof, Ilhave-hereuntoset 1 "HUGOEROMAINDER.

ods when the anode is de-energized, whereby means whereby oscillations are applied betweensaid grid and filament'during the generation of oscillations, and means making said grid positive with respect to said cathode operative at least a portion of the time dur ng wh ch oscillatlons are not being generated.

12. In a high frequency carrier wave system, a source of oscillations, a vacuum tube having a cathode, grid, and anode, a frequency determining circuit comprising as an element thereof the capacity between the grid and other electrodes of said tube, means for impressing signal frequency potentials between the cathode and grid of said tube whereby a grid heating current results, and

means for preventing the application to said input circuit of said signal potentials and for simultaneously applying thereto a differently characterized potential capable of producing the same heating effect on said grid.

13. The method of improving the stability of operation of a three-electrode vacuum tube which consists in maintaining the grid of the tube at a substantially uniform temperature I by applying supplemental heating current thereto during non-operating intervals only.

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